The use of catalytic converters to reduce the output of pollutants from the exhaust of vehicles powered by internal combustion engines has brought out the need to insulate the exhaust pipe leading to the converter in order to deliver gases at high temperatures. Because the catalytic converter does not begin to catalyze the reaction in the converter until its light-off temperature is reached, by reducing the heat loss of the exhaust gases delivered to the catalytic converter the period during which pollutants are released is also reduced, resulting in a lesser total amount of pollutants released to the atmosphere.
Efforts to insulate the exhaust pipe have centered on the use of high temperature insulation on the pipe, with the insulation held in place by another larger pipe concentrically spaced from the exhaust pipe. To withstand the high temperatures of the exhaust gases refractory fiber is the preferred insulating material. To reduce the thermal mass of the exhaust pipe, which also contributes to maintaining the high temperatures of the exhaust gas and thus aids in reducing the catalytic light-off period, very thin metal tubing has been proposed for use as the exhaust pipe.
Exhaust pipe structure of this type is difficult to attach to the manifold or other elements of the exhaust system. The fragile nature of the refractory fiber insulation and the thin metal tubing give the pipe little resistance to crushing or deformation by clamps designed to hold the ends of the pipe in place through high pressures. If the pipe is not securely held in place, however, the vibration to which it would be subjected in time degrades the refractory fibers, reducing them to dust-like particles and destroying their insulating value. Further, the attachment should prevent the escape of gases from the inner tube into the insulation. This can readily occur at the end of the pipe, resulting in the outer pipe being exposed to the hot gases and increasing the heat loss from the exhaust pipe.
It is thus an object of the invention to provide an exhaust pipe attachment means which does not damage the pipe or the insulation but nevertheless holds the pipe securely in place against vibration. It is also an object to prevent exhaust gases from entering the insulation at the end of the pipe.